Text size in X

I can't find anything in the Xterm menu to control the size of
text in my terminal window. Xterm has a command-line option
by which you can control the font when you start it up, but
I want to change it in an existing window.

In Evolution, I can use View > Text Size, but it has no
effect.

I am running with a high resolution screen, which gives me
the sizes I want in toolbars and menus; but then I need to
increase the size of the application text accordingly.

Suggestions?

Thanks,

Vic